Thank you so much for your prompt reply. I have infact tried to replace the comma within the separator tags with semi-colon - that did not work and then had gone through all the discussions and FAQs, but still I could not get a solution. hen only I tried to
replace with <xsl:text>;</xsl:text>

Please correct me if I am wrong but if you will notice, the output by the default ACM-Citation Sequence does not show the comma between the citations, i.e., it appears as [12] [13] [5] [14] [1] instead of appearing as [12], [13], [5], [14], [1].

It has also proved to be effective in protein classification [11] and gene expression-based cancer subtype discovery [12] [13] [5] [14] [1].

Same is the case of the output by Nature: appears as 12 13 5 14 1 instead of 12, 13, 5, 14, 1.

It has also proved to be effective in protein classification 11 and gene expression-based cancer subtype discovery 12 13 5 14 1.